The present invention relates to a gas insulated cable particularly of the variety used for transmitting high or very high voltages.
Gas insulated cables are usually of tubular construction, and a conductor or conductors are positioned therein by means of spacers in particular relation with respect to each other as well as to the tubular jacket. In a typical case, an inner conductor is provided as a tube or is constructed from stranded wires. This conductor is held concentrically inside of a metal tube of correspondingly large diameter whereby disc shaped, funnel shaped or other kinds of elements provide for the supporting function. The interior of the outer tube, i.e. the space between the latter and the inner conductor, is filled with a gas such as SF.sub.6, which is the most commonly used insulating gas today.
The insulative strength of such a coaxial cable depends upon the quality of the insulation in general as well as upon the geometry of the cable, i.e. the radial distance between inner conductor and outer tube. The gas insulation as such is quite poor from a general point of view, so that the transmission of high voltages usually require the cables to have larger diameters. Not only are large diameter cables rendered more expensive than small ones on account of increased cost in manufacturing, but handling of thick cable such as transportation, installation, etc., is cumbersome and compounds the problem.
The insulative strength of such a gas filled cable can be increased to some extent by increasing the gas pressure. However, the relation between these parameters requires a rather steep increase in pressure to obtain a modest increase in insulative strength. Moreover, a pressure increase is often accompanied by an increase in impurities which reduce the insulative properties of the cable. Thus, additional steps have to be taken to clean the gas before filling the cable. Another problem is to be seen in the fact that the liquifying temperature or boiling point of SF.sub.6 increases with pressure to such an extent that it readily approaches the operating temperature of the cable. It can thus be seen that increasing the gas pressure is of limited value for enhancing insulation of a gas filled cable.